Demand is mounting for diesel engines mainly in Europe because of their high fuel efficiency. Because particulate matter (hereinafter referred to simply as “PM”) and NOx contained in exhaust gases discharged from diesel engines largely affect human health, various investigations have been conducted to reduce the amounts of PM and NOx. As an exhaust gas-cleaning apparatus for reducing the amounts of PM and NOx in an exhaust gas, JP2000-303826 discloses, as shown in FIG. 4, an exhaust gas-cleaning apparatus comprising an oxidation catalyst 3, a filter 4 and a selective catalytic reduction (SCR) catalyst 5 in this order from upstream in an exhaust gas path 2 connected to a diesel engine 1, a valve 6a communicating with a reducing agent reservoir 6b being provided between the filter 4 and the SCR catalyst 5.
JP2002-502927A discloses an improved SCR system for treating a combustion exhaust gas containing NO and PM, which comprises a particulate matter trap (filter) comprising a catalyst for effectively oxidizing at least part of NO in NOx to NO2 and burning at least part of PM at relatively low temperatures in the presence of NO2, a reservoir of a nitrogen compound (ammonia or urea) as a reducing agent, and an SCR catalytic converter for converting NOx to N2 by flowing a mixture of an NO2-rich exhaust gas and the nitrogen compound as a reducing agent there through, in this order. The SCR is to selectively reduce NOx to N2 by causing a high-temperature exhaust gas to pass through a catalyst in the presence of a nitrogen compound (ammonia or urea) as a reducing agent.
JP2002-502927A describes that an oxidation catalyst supported by the filter disposed upstream of the SCR catalytic converter is effective to increase the conversion ratio of NOx to N2 in the SCR catalytic converter, that the oxidation catalyst is preferably a platinum catalyst supported by a ceramic or metal through-flow honeycomb, that the filter is preferably a ceramic wall-flow filter, and that the SCR catalyst is preferably a catalyst of V2O5/WO3/TiO2 supported by a through-flow honeycomb.
JP2005-296861A discloses a filter for cleaning an exhaust gas from a diesel engine, which comprises porous cell walls partitioning flow paths and carrying an oxidation catalyst, an exhaust gas flowing into inlet-side flow paths entering into adjacent flow paths opening at the outlet side via the porous cell walls and being discharged therefrom, and the amount of a catalytically active component per a unit area of the porous cell walls being larger on the outlet side than on the inlet side. An SCR catalytic converter is disposed downstream of this filter. JP2005-296861A describes that with the SCR catalytic converter disposed downstream of a filter carrying an oxidation catalyst, and with the amount of the oxidation catalyst carried by the porous cell walls larger on the outlet side than on the inlet side, NOx can be efficiently removed even when the exhaust gas is at a low temperature. It further proposes that a catalyst and a heater are disposed upstream of the filter as means for burning PM at lower temperatures.
However, the temperature of an exhaust gas largely differs between when the diesel engine is fully operated and when it is idling. In a case where start and stop are repeated, a low-temperature exhaust gas is sometimes discharged. When the temperature of the exhaust gas becomes lower, the temperature of an SCR catalytic converter downstream of a filter is lowered, resulting in decrease in the conversion ratio of NOx to N2.
JP2002-250220 discloses an exhaust gas-cleaning apparatus for diesel engines comprising a particulate filter and a NOx catalyst in one casing, a urea-spraying nozzle being open on an inner wall of the casing between the particulate filter and the NOx catalyst. However, because there is inevitably a small gap between the particulate filter and the NOx catalyst for the purpose of making the exhaust gas-cleaning apparatus smaller, it has been found that the urea-spraying nozzle open on the inner wall of the casing fails to have urea uniformly mixed with an exhaust gas discharged from the particulate filter.